A bandgap reference (BGR) circuit is an essentially temperature independent voltage reference circuit widely used in integrated circuits (ICs). The principle of the bandgap voltage reference is to balance the negative temperature coefficient of a pn junction with the positive temperature coefficient of the thermal voltage, Vt=kT/q where T is the temperature, k the Boltzmann constant, and q is the electronic charge, to reduce reference voltage variation with temperature by having circuitry which sums a Proportional to Absolute Temperature (PTAT) current in a first branch and a Complementary to Absolute Temperature (CTAT) current in a second branch.
The BGR thus generates an essentially fixed (constant) voltage that is largely invariant irrespective of power supply variations, temperature changes and the loading on the BGR. The BGR typically has an output voltage around 1.25 V, which is nearly the voltage corresponding to the theoretical 1.22 eV bandgap energy of silicon at 0 K.
The BGR circuit has two stable states, an off state, which does not output a reference voltage (its off-state), and an operational state (its on-state), which provides the desired reference voltage. When power is first applied to a BGR circuit, the BGR enters its off-state, in which no current initially flows through the BGR circuit. The BGR remains in its off-state until another circuit referred to as a startup circuit forces it to transition to its on-state. Once the on-state has been established, the startup circuit is electrically disconnected from the BGR circuit so that the startup circuit no longer influences the operation of the circuit being served by the BGR circuit.